The present invention relates to a switching power supply control circuit and more particularly to a switching power supply control circuit that operates a switching power supply system by switching its state between a normal state and a stand-by state.
At present, a switching power supply system that supplies power to various kinds of electric devices generally supplies power in two modes, a normal mode in which electric devices as objects of supplying electric power are made to carry out normal operations and a stand-by mode in which the electric devices are brought into stand-by states.
FIG. 6 is a circuit diagram showing an example of a switching power supply circuit forming a switching power supply system. The switching power supply circuit of the example shown in FIG. 6 is formed of two stages of a first converter 10 and a second converter 20. The first converter 10 is a PFC (Power Factor Control) step-up converter in which a PFC circuit 11 controls a switching device to step-up a full-wave rectified AC input voltage to convert it to a DC voltage. The output of the first converter 10 becomes an input to the second converter 20. The second converter 20 is a DC to DC converter which is shown here as an example of a half-bridge current resonance converter here. The second converter 20 supplies energy onto the output side through an insulating transformer.
In the second converter 20, the current in a light emitting element 24 in a photo coupler PC1 varies according to an output voltage. A photo-signal emitted from the light emitting element 24 in the photo coupler PC1 is detected by a photodetector 22 in the photo coupler PC1 to be a feedback voltage (an FB terminal voltage) to a control IC 21 provided in the second converter 20. The electric potential of the FB terminal of the control IC 21 is pulled-up to a high potential side by an element such as a pull-up resistor not shown. The control IC 21 has a VCO (Voltage Controlled Oscillator) 21a, a control unit 21b and a starting circuit (START) 21c. The VCO 21a and the control unit 21b of them are provided in a switching power supply control circuit (not shown here) forming a part of the control IC 21. The switching power supply control circuit will be explained later. According to the variation in the FB terminal voltage, the oscillation frequency of the VCO 21a, connected to the FB terminal in the control IC 21, is made varied, by which control is carried out so that an output voltage of the switching power supply circuit is made constant. The START 21c is connected to the output side of the first converter 10 through a VH terminal and has a function of clamping a VCC voltage, supplied by rectifying and smoothing the output voltage of an auxiliary winding of the insulating transformer, at a specified voltage of Vsus by a current supplied from the VH terminal as a measure for stopping reduction in the VCC voltage. The second converter 20 is a current resonance type converter and energy which the second converter 20 can supply to the output side depends on the voltage applied across a resonance capacitor Cr. Hence, when the output voltage of the first converter 10 is low, an amount of the energy which the second converter 20 can supply to the output side is restricted.
Switching between an operating state in a normal mode and that in a stand-by mode is carried out according to instructions outputted from a stand-by instruction circuit 30 operated by another power supply such as a battery. The stand-by instruction circuit 30 ordinarily instructs a normal mode for a rated load (heavy load) by a high potential signal (hereinafter referred to as “H signal”) and instructs a stand-by mode for a light load by a low potential signal (hereinafter referred to as “L signal”). In the normal mode, a light emitting element 25 in a photo coupler PC2 is made turned on. While, in the stand-by mode, the light emitting element 25 in the photo coupler PC2 is made turned off. The reason for making the light emitting element 25 in the photo coupler PC2 turned off in the stand-by mode is for reducing power consumption in the stand-by mode. The light signal emitted from the light emitting element 25 in the photo coupler PC2 is received by the photodetector 23 in the photo coupler PC2 to be transmitted to the control IC 21 via an STB terminal. The control IC 21 carries out switching of the operating states according to the voltage at the STB terminal.
At the operation in a stand-by mode, for reducing the power consumption in the stand-by mode, the switching operation of the first converter 10 is further made stopped. While, the second converter 20 is made to carry out an intermittent operation or carry out control such as PFM (Pulse Frequency Modulation) control or PWM (Pulse Width Modulation) control with a low frequency to reduce a switching loss. The operation and termination of the switching of the first converter 10 are switched by an inner stand-by signal outputted from a PFC_EN terminal of the control IC 21 in the second converter 20.
In this way, a signal for the feedback of an output voltage detecting signal and an instruction signal for switching operation modes are respectively transmitted by using different photo couplers.
Thus, there is proposed a switching power supply system in which a feedback signal of an output voltage detection signal and instruction signals for switching between operation modes are transmitted by one photo coupler (JP-A-2001-86745, for example). In such a switching power supply system, an output signal of the photo coupler is used as a feedback signal of an output voltage detection signal in a normal mode. When an instruction is given for switching from a normal mode to a stand-by mode, a shunt regulator connected to the photo coupler for controlling an amount of light emission is made short-circuited to force the photo coupler to emit light with the maximum amount. The sate is equivalent to a state with an abnormally high output voltage, in response to which the state of the system is made to transfer from the normal state into a stand-by mode.
Moreover, there is proposed a switching power supply system in which two photo couplers are provided for controlling an output voltage and for instructing switching between a normal mode and a stand-by mode, respectively. In the switching power supply system, the photo coupler for instructing switching between a normal mode and a stand-by mode has a collector of a phototransistor on the primary side connected to a half-wave rectifier circuit (see JP-A-2002-44942, for example). This makes a switching device turned-off as being in a stand-by mode only in a period in which a voltage from the half-wave rectifier circuit is applied to the collector of the phototransistor to permit power consumption reduction in the stand-by mode. For bringing the system into a stand-by mode, the phototransistor on the primary side in the photo coupler for instructing switching between a normal mode and a stand-by mode must be made turned-on. For this purpose, also in this case, the photo coupler for instructing switching between a normal mode and a stand-by mode must be forced to emit light with a large amount.
Here, a switching power supply control circuit forming a part of the control IC 21 will be explained. FIG. 7 is a circuit diagram showing an example of a related switching power supply control circuit. In FIG. 7, double circles in FIG. 7 represent input and output terminals. Signs attached to the input and out terminals correspond to those attached to input and output lines of the control IC 21 shown in FIG. 6. The FB terminal, the STB terminal, and OUTH and OUTL terminals are terminals for a feedback signal of an output voltage detection signal, an instruction input signal instructing an operation mode and output signals corresponding to instructed operation modes, respectively. A switching power supply control circuit 90 has a comparator 91, a VCO 92 controlling a switching power supply, and a control unit 93. The VCO 92 and the control unit 93 correspond to the VCO 21a and the control unit 21b, respectively, which are shown in FIG. 6. The comparator 91 compares the voltage of an instruction input signal inputted from the STB terminal with a specified operation mode decision reference voltage VthSTB. In general, the voltage of an instruction input signal for a normal operation mode is made higher than the VthSTB, while the voltage of an instruction input signal for a stand-by mode is made lower than the VthSTB. The comparator 91 outputs an H signal in the normal mode and an L signal in the stand-by mode to the control unit 93. The VCO 92 varies an oscillation frequency according to a feedback signal inputted from the FB terminal. The control unit 93 determines an operation mode according to the output signal of the comparator 91 to output switching signals based on the output of the VCO 92 from the OUTH terminal and the OUTL terminal in a normal mode and output signals for stopping a switching operation from the OUTH terminal and the OUTL terminal in a stand-by mode.
In general, there are two kinds of operation modes for starting up operation modes when starting up a switching power supply system by turning-on a power supply switch, a stand-by starting up mode and a rated load starting up (heavy load starting up) mode. In the related switching power supply system, however, there was a problem that the system sometimes can not be brought into operation in a normal mode after being made to start up in either one of the two kinds of modes.
The stand-by starting up mode is a mode in which a switching power supply system is made to start up in a stand-by mode with a light load when a main power supply is turned-on. After the system is made to start up in the stand-by starting up mode, when it is detected that a normal mode is instructed by an instruction input signal, the operation mode is made switched to the normal mode with a rated load.
The rated load starting up mode is a mode in which a switching power supply system is made to start up in a normal mode with a rated load. Here, an operation sequence in the stand-by starting up mode will be explained. FIG. 8 is a diagram showing variations with time in voltages and currents at various positions in a switching power supply circuit provided with the related switching power supply control circuit 90 shown in FIG. 7 in a starting up operation state in the stand-by starting up mode.
Turning-on of a main power supply at a time t1 results in an increase in a VCC voltage inputted to the control IC 21 shown in FIG. 6 by the operation of the START 21c in the control IC 21. The increased VCC voltage reaches a UVLO (Under Voltage Lock Out) on-voltage (VCCON) at a time t2. With the VCC voltage reached the UVLO on-voltage, the control IC 21 starts its switching operation. Because of the starting up operation mode being the stand-by starting up mode, at the time t2, the stand-by instruction circuit 30 outputs an L signal, by which the light emitting element 25 in the photo coupler PC2 is made turned-off. Therefore, the photodetector 23 in the photo coupler PC2 is not operated to be also made turned-off to cause the voltage at the STB terminal of the control IC 21 to be also in the state of the L signal. Thus, the switching operation of the second converter 20 is made started from a state in the stand-by mode. Since the operation is in the stand-by mode, the first converter 10 is in a state with its switching operation being stopped. The switching operation made started by the control IC 21 increases current consumption (stand-by current) to once reduce the VCC voltage. However, because of an electrical supply started thereafter from an auxiliary winding of the insulating transformer shown in FIG. 6, the VCC voltage increases again. This also increases the output voltage of the second converter 20 to make the voltage exceed a voltage that enables the photo couplers to be turned-on. At a time t3 at which the second converter 20 is brought into this state, the output of the stand-by instruction circuit 30 is switched to an H signal, by which switching from the stand-by mode to a normal mode is instructed. This makes the light emitting element 25 in the photo coupler PC2 turned-on to make the photodetector 23 in the photo coupler PC2 turned-on to bring the voltage at the STB terminal also exceeds the operation mode decision reference voltage VthSTB. Then, the control IC 21 switches the mode of the switching operation of the second converter 20 from the stand-by mode to a normal mode to further start the switching operation of the first converter 10. A resulting increase in the output voltage of the first converter 10 enables the second converter 20 to supply a rated load current, which increases the output voltage of the second converter 20 up to a control voltage to be made stabilized.
Next to this, an operation sequence in a rated load starting up mode will be explained. FIG. 9 is a diagram showing variations with time in voltages and currents at various positions in a switching power supply circuit provided with the related switching power supply control circuit 90 shown in FIG. 7 in a starting up operation state in the rated load starting up mode. The operation in the period from the time t1 at which the main power supply is turned-on to the time t2 at which the VCC voltage reaches the VCCON is the same as that in the stand-by starting mode shown in FIG. 8. Because of the rated load starting up mode, at the time t2, the output of the stand-by instruction circuit 30 is given as an H signal. However, the low output voltage of the second converter 20 makes the light emitting element 25 in the photo coupler PC2 impossible to be turned-on. This also makes the photodetector 23 in the photo coupler PC2 impossible to be turned-on to cause the voltage at the STB terminal in the control IC 21 to be provided as an L signal. As a result, the mode of the switching operation of the second converter 20 becomes a stand-by mode, in which an inner stand-by signal of stopping the switching operation of the first converter 10 is outputted from the PFC_EN terminal of the control IC 21. From the first converter 10 with its switching operation being thus made stopped, no increased voltage is supplied to the second converter 20. Therefore, no sufficient energy (current) can be supplied onto the output side of the second converter 20. At this time, the switching power supply circuit, being in a state with a rated load, allows no output voltage to be made increased when its load current is larger than the output current that can be supplied by the second converter 20. Once the switching power supply circuit enters in the state, although the stand-by instruction circuit 30 is outputting an H signal, the light emitting element 25 in the photo coupler PC2 can not be made turned-on to make the switching power supply circuit impossible to return from the stand-by mode to a normal mode. In FIG. 9, the reason that the VCC voltage is held at a voltage Vsus is due to the function of a clamp circuit in the START 21c in the control IC21.
Thus, there is a problem in that presence of a load being heavy to some extent from the beginning as in the rated load starting up mode makes at turning-on of the power supply the switching power supply circuit caught in a vicious cycle of causing no output voltage to be increased, enabling next no photo coupler to be turned-on, making then the switching power supply circuit brought in no normal operation mode (no rated load condition) and thus causing no output voltage to be increased.
Letting a state in which the photo couplers are made turned-off be a state in the normal operation mode, the switching power supply circuit can be brought into the normal operation mode even at turning-on of the power supply. In this case, however, photo couplers must be made turned-on in a stand-by mode. Thus, the charges in an output capacitor are consumed in the turned-on photo couplers, which consumption gradually reduces the output voltage. This makes it impossible for the switching power supply circuit to maintain the state in the stand-by mode for a long time. The same is true for the case of forcing a photo coupler to emit light when an instruction is given to switch the mode to the stand-by mode as in the invention disclosed in each of JP-A-2001-86745 and JP-A-2002-44942. In each of the inventions, a photo coupler is made to emit light so that a control circuit is to detect an output voltage as being in an abnormally high state, by which the converters are not operated. This causes no charges to be supplied to the output capacitor at all to lead to remarkable reduction in the output voltage when the state in the stand-by mode is made prolonged. Thus, there is a possibility of making a photo coupler impossible to be forced to emit light to make the switching power supply circuit impossible to maintain a state in the stand-by mode or impossible to drive a load when the stand-by mode is cancelled.
The same problem arises even though a switching power supply system is one formed of one converter (such as a flyback power supply, for example) rather than one formed of two stage converters as shown in FIG. 6. That is, in the case where the converter is one that carries out a stop of its switching operation, an intermittent operation or PFM control at a low frequency when in a stand-by state, since the output current of the converter is limited in a stand-by mode, when a load current is larger than the current which can be supplied by the converter once brought into the stand-by mode at the start up in a rated load start up mode, the same defect as above is caused.
The invention was made in view of such points with an object of providing a switching power supply control circuit which can stably start up with an instructed mode at turning-on of a power supply.